The present invention relates to electric motors and, more particularly, to linear electric motors.
Many processes require displacing a load along a linear axis to perform a desired function. Typical conventional electric motors employ a rotor driven in rotation about an axis by interacting magnetic fields. As a consequence, means are required to convert the rotational torque of the motor into a linear force. Such conversion means include, for example, ball and screw, and rack and pinion. The conversion means, besides contributing to mechanical inefficiency, adds inertia and backlash (or windup) to the apparatus.
In high-precision linear positioning devices such as, for example, positioning tables of a type disclosed in my U.S. Pat. No. 4,013,280, I discovered that windup of a conventional ball-screw drive interfered with attaining high levels of precision. In addition, the inertia added to the system by the presence of the ball-screw drive reduced the acceleration available to drive the table to high linear rates.
One solution to the backlash or windup problem of a conventional ball-screw drive is disclosed in my U.S. Pat. No. 4,378,709, wherein a conventional ball-screw drive is replaced with a bar affixed to a positioning table and clamped between a drive roller and one or more backup rollers. A drive motor, directly driving the drive roller, moves the bar with the affixed positioning table without backlash from intervening elements.
While the last-referenced patent is effective for reducing windup, it is only partly effective for reducing inertia. The bar, motor, drive roller and backup rollers, although potentially less massive than the elements of a ball-screw drive, nevertheless add mass to the system which must be driven along with the positioning table.
Numerous linear motors are disclosed in the prior art for direct linear drive. These include induction, variable reluctance, and stepper motors. For high-precision positioning tables, I discovered that a permanent-magnet, DC linear motor was highly desirable. Such a device, disclosed in my U.S. Pat. No. 4,560,911, employs a linear armature having a stationary toothed structure containing a plurality of armature coils and a movable structure containing a plurality of permanent magnets facing the armature. Commutator brushes, affixed to the movable structure, contact a stationary commutator to apply properly phased power to the armature coils. In one embodiment of that invention, a pair of stationary power pickup rails are disposed parallel to the commutator and a pair of power pickup brushes are affixed to the movable structure for obtaining the energizing electricity. Alternatively, the electricity may be supplied by pendant cable without requiring power pickup rails and brushes.
The practical development of the above permanent magnet DC linear motor was aided by the availability of high-strength permanent magnets made of, for example, samarium cobalt. Such permanent magnets are light in weight and have extremely high magnetic field strengths. These properties are ideal for positioning light loads at high accelerations, as desired in high-precision positioning tables.
I have discovered that advantage can be taken of the high magnetic field strength of available permanent magnets to increase the linear force available from permanent magnet DC linear motors beyond the few pounds required by a high-precision positioning table. As is well known, the force developed by the interaction of an electric current and a magnetic field is proportional to the product of the armature current and the magnetic field strength. High forces thus require high armature current. Resistive losses in the windings raise the temperature in the armature. The temperature rise must be limited to a value below which heat damage to armature insulation can occur.
The armature of a linear motor is a natural heat sink. The large stationary mass of magnetic iron about which the armature coils are wound is capable of absorbing substantial heat while maintaining a reasonable temperature. In my above-referenced linear motor, the armature is relatively long compared to the length of the movable element. The commutating scheme energizes only those armature coils within the magnetic influence of the permanent magnets in the movable element. An application of such a linear motor which positions the movable element in generally random locations along the armature tends to distribute the heat along the armature. This makes available the relatively large structure of the armature for discharging the heat.
Some applications prevent random deposit of heat along an armature. For example, the high current experienced in accelerating a linear motor may often occur over a small portion of the length of the armature. Thus, even though the magnetic iron in the armature is available as a heat sink, the heat is concentrated in a small portion of the iron, giving rise to unacceptable temperature rise in that portion.
In my U.S. Pat. application Ser. No: 638,488, now U.S. Pat. No. 4,625,132, I disclose a linear motor with a U-shaped frame having a flexible seal closing the arms of the U shape. The movable element within the U-shaped frame is connected to a load through a plate passing through the flexible seal. A coolant fluid such as, for example, air, is injected into one or both ends of the U-shaped frame at a positive pressure with respect to the ambient gas pressure. The coolant fluid flows toward the portion of the armature having coils energized for interaction with the permanent magnets of the movable element. The flexible seal keeps the coolant fluid within the U-shaped frame and at least partly seals the plate. Controlled coolant leakage about the plate permits discharge of the heat from the U-shaped frame in the vicinity of the energized armature coils, which is, of course, precisely where the coolant is most desired.
A linear motor with seal, as disclosed above, has the further advantage in a dirty factory environment of preventing the entry of dirt into the U-shaped frame. Even in the absence of a positive pressure of coolant, the seal excludes dirt. With a positive coolant pressure, even greater cleanliness can be maintained within the linear motor.
For the generation of even greater force, I have discovered that even a moderate flow of a liquid coolant in thermal contact with the armature is capable of maintaining an armature temperature rise within acceptable limits. One embodiment of a linear motor with liquid coolant flow in its armature is disclosed in U.S. patent application Ser. No. 859,915 now abandoned.
The combination of high-magnetic-field-strength permanent magnets with the new ability to cool the armatures of permanent magnet DC linear motors now permits the design of linear motors capable of sufficient force to enter applications never before contemplated for such devices.
Modern permanent magnets facing an armature containing magnetic material exert a high attractive force therebetween. Such high attractive force must be resisted by the structure supporting the movable element. In the case of the above-referenced positioning table, the bearings supporting the table slide for linear motion along its axis are also employed for supporting a movable element affixed thereto. In my U.S. Pat. No. 4,505,464, I disclose a positioning table in which the magnetic attraction of a permanent magnet linear motor is employed for pre-loading the bearings thereof. Thus, rather than being a problem, the magnetic attraction in a permanent magnet linear motor becomes an advantage.
An alternative approach for dealing with the bearing loading imposed by magnetic attraction in a linear motor is disclosed in my U.S. Pat. No. 4,595,870, in which the magnetic attractive forces exerted in a first direction by a first set of permanent magnets are balanced by equal magnetic attractive forces exerted in the opposite direction by a second set of permanent magnets.
A further alternative approach for dealing with the bearing loading imposed by magnetic attraction is disclosed in my U.S. patent application Ser. No. 887,383, now U.S. Pat. No. 4,749,921, where in the armature is formed of non-magnetic material. Thus, the static magnetic attractive forces in conventional permanent magnet linear motors are eliminated.